Ma-m2t spinel solid solution enhanced magnesiium oxide-based ceramic foam filter and preparation method therefor

ABSTRACT

An MA-M2T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter and a preparation therefor. The preparation method comprising: 1) preparing a ceramic slurry having a solid content of 60%-70% by dosing 15%-25% by mass of a nanometer alumina sol, 0.8%-1.5% by mass of a rheological agent, and the balance magnesium oxide ceramic powder comprising a nanometer titanium oxide sintering aid, and then adding deionized water and ball milling to mix until uniform, and then vacuum degassing the mixture; 2) soaking a polyurethane foam plastic template into the ceramic slurry, squeezing by a roller press the polyurethane foam plastic template to remove redundant slurry therein to make a biscuit, and drying the biscuit by heating it to 80° C.-120° C.; 3) putting the dried biscuit into a sintering furnace, elevating the temperature to 1400° C.-1600° C. and performing a high temperature sintering, cooling to the room temperature with the furnace to obtain the magnesium oxide-based ceramic foam filter.

TECHNICAL FIELD

The present invention relates to a magnesium oxide-based ceramic foamfilter and a method for preparing the same, in particular, an MA-M₂Tspinel solid solution-reinforced magnesium oxide-based ceramic foamfilter and a method for preparing the same, and it relates to the fieldof metal materials and metallurgy. The filter prepared in accordancewith the present invention is especially suitable for filtration andpurification of a melt of magnesium and alloys thereof, but can also beused for filtration and purification of a melt of aluminum and alloysthereof.

BACKGROUND ART

Magnesium is chemically active and can react easily with oxygen,nitrogen and water vapor in the process of casting or handling, and theformed products may be left in magnesium, which would affect theinternal quality of the article and degrade the properties of the same.According to the variety and nature of inclusions in a magnesium alloy,the inclusions are generally divided into two categories: metalinclusions and non-metal inclusions. 1) Metal inclusions: In the processof original magnesium production and subsequent handling of a magnesiumalloy, some pure metals or metallic compound inclusions would beinevitably introduced, and they would present in the matrix or at thegrain boundary of the magnesium alloy in the form of particles orclusters, including primarily pure metal α-Fe particles, Mn—Fe metalliccompounds, such as, for example, (Fe, Mn)₃Si, (Fe, Mn)₅Si₃, amongothers; 2) non-metal inclusions: Non-metal inclusions in an magnesiumalloy are primarily nitrides and oxides of magnesium, such as, forexample, MgO, Mg₃N₂, among others; refining agents added in the processof smelting of magnesium alloy, primarily chlorides, such as KCl, NaCl,MgCl₂, among others; since the fluxing agents in the refining processcannot be removed completely, some small amount of residual fluxingagents may be present in the magnesium melt, resulting in fluxing agentinclusions in the magnesium metal. The floating oxide inclusions will bepushed to the grain boundaries by the crystallizing front, and theinclusions will be generally present at the grain boundaries of themagnesium alloy in the form of films, particles, or clusters. Statisticsshow that MgO accounts for more than eighty percent of all inclusions ina magnesium alloy, and is distributed in the form of films, particles,and clusters. The inclusions generated in the process of melting andcasting of a magnesium alloy will not only seriously degrade themechanical properties and anti-corrosion performance of the alloy, butalso degrade the machining quality and surface quality of anodicoxidation treatment thereof. With respect to a die-casting magnesiumalloy, the content of oxides in the form of films and particles insidethe alloy needs to be controlled respectively below 100 cm²/Kg and 100mm³/Kg to satisfy the normal operating requirements. Therefore, thepurification process has been critical in the melting and castingprocess for removing inclusions in a magnesium melt in order to increasethe purity thereof.

Melt purification process can be classified into two categories: fluxpurification and non-flux purification. Due to high inclusion removingefficiency, low cost, convenient operation, flux purification processhas become a commonly used purification process in the manufacture ofmagnesium alloys. However, flux purification also exhibitsdisadvantages, such as, for example, increase of metal loss, fluxinclusions, inability of degassing, among others. In particular, when arear earth magnesium alloy is smelted and refined, the flux will alsoconsume a large amount of rare earth elements in the alloy. Non-fluxpurification process can not only compensate for the deficiencies offlux purification process, but also exhibit excellent purificationresults, and thus is an important melt purification process being usedand developed. Various non-flux purification technologies, such asfiltration purification, spinning spraying purification, electromagneticpurification and ultrasonic wave treatment, among others, have beendeveloped. Compared to simple metal net melt filtration, ceramic foamfilters comprising particular 3-D porous ceramic structures have verygood filtration effects with respect to inclusion particles in an alloymelt through filter cake effect, adsorption effect and rectificationeffect due to their high porosity (70%-90%), strong adsorbability,chemical corrosion resistance, among others. Ceramic foam filtrationmethod can not only filter out fine inclusion particles as small as 10μm-20 μm in an alloy melt, but also can filter out liquid fluxinclusions that cannot be filtered out by a general filtering medium.

U.S. patent documents U.S. Pat. No. 3,962,081 A (Ceramic foam filter),U.S. Pat. No. 4,024,212A (Ceramic foam and method of preparation) andChinese patent document CN 103,787,691 A (A method for preparing analuminum oxide ceramic foam) disclose methods of using Al₂O₃. ZrO₂, SiCor SiO₂—based ceramic foams for filtering inclusions in aluminum alloyor steel or iron melts; however, the standard free enthalpy of formationof MgO is very low, and the highly active Magnesium melt is very proneto react with the matrix material of the ceramic foam in accordance withthe equations (1)-(4) and dissolved quickly, and accordingly blocks thefiltration holes, or melts and corrodes and enters the melt of magnesiumor an alloy thereof, as hazardous components. Thus, the ceramic foams ofcurrent materials are not suitable for filtration of magnesium and thealloys thereof.

3Mg_((l)+)Al₂O_(3(s)=)3MgO_((s)+)2Al_((l))   (1)

2Mg(l)+ZrO₂(s)=2MgO(s)+Zr(s)   (2)

6Mg(l)+4Al(l)+3SiC(s)=3Mg₂Si(s)+Al₄C₃(s)   (3)

4Mg(l)+SiO₂(s)=2MgO(s)+Mg₂Si(s)   (4)

MgO is with a NaCl structure, cubic system, lattice constant being 0.411nm, and is an ionic bond compound, melting point being 2852° C., whichis much higher than commonly used Al₂O₃ (2054° C.) and SiO₂ (1650±50°C.). Therefore, articles formed of magnesium oxide exhibitcharacteristics such as good chemical stability, high resistivity andanti-erosion abilities to metals, slag and alkaline solutions. Comparedto commonly used ceramic materials, MgO exhibits good high temperaturechemical stability against magnesium and the alloys thereof, does notreact with the flux inclusions comprised of molten chlorides andfluorides, and has a relatively small wetting angle with respect to theflux inclusions and thus readily adsorb the same in magnesium melt.Accordingly, MgO ceramic foams are ideal materials for smelting,refining and purification of magnesium alloys.

It would be a necessary and most critical step of preparing ceramicmaterials to perform sintering below the melting point of the oxidecomponents, and the sintering and grain growing process at a hightemperature will decide the micro-structure and performance of a ceramicmaterial. Chinese patent documents CN 1,011,306 B (Pure Magnesium OxideCeramic Foam Filter and Process of Preparing the Same), CN 101,138,691A(Method of Preparing Magnesium Ceramic Foam Filter for Casting) discloseusing pure magnesium oxide as the starting material to produce ceramicfilters. Since MgO has a very high melting point and thermal expansioncoefficient (13.5×10⁻⁶/° C.), the sintering of the same is difficult(the sintering temperature should not be lower than 0.8 of its meltingpoint) and the thermal shock resistance is poor, which limit the use anddevelopment of MgO ceramic foams.

Researches show: when the sintering temperature in a ceramic sinteringprocess is decreased by 100° C., the heat consumption for unit productwill be reduced by 10%. An important technical means of decreasing thesintering temperature of MgO ceramic foam is addition of a sinteringaid. When V₂O₅ powder is added, MgO and V₂O₅ will form a liquid phasehaving a formula similar to Mg₃V₂O₈ at 1190° C. to facilitate thesintering and decrease the sintering temperature of a MgO ceramic foamsignificantly. However, in use, V₂O₅ will cause damages to respiratorysystem and skin, and accordingly strict restrictions are imposed on theoperations thereof. Same to V₂O₅, cobalt oxide is also a good lowtemperature sintering aid, but the use is restricted due to its highlytoxic nature and rare resources. Fluorides are commonly used strongsolubilizer and mineralizer in ceramic industrial sintering. Chinesepatent documents CN 100,536,986 C (Magnesium Oxide Ceramic Foam Filter),CN 1,473,947 A (Ceramic Foam for Purification of Magnesium and MagnesiumAlloys) and CN 101,785,944 B (Method of Preparing Magnesium Ceramic Foamfor Filtration of Magnesium and Magnesium Melt) disclose adding fluorite(melting point: 1423° C.) and magnesium fluorite (melting point: 1248°C.), wherein the solid solution of the fluoride during sintering wouldincrease the lattice distortion of the magnesium oxide matrix, and thefluoride would be prone to form a liquid having a lower melting point,so as to decrease the sintering temperature of the magnesium oxideceramic. However, in the sintering process, fluorine in the fluorideswould be bonded with Si, Al, Fe and Ca, and most of them (account forabout 70% in production of tiles) is volatilized in gas form, whichwould not only erode the ceramic body and degrade the quality of thesintered ceramic, but also, more seriously, it would be discharged tothe atmosphere and cause fluoride contamination. Fluorides can enterhuman body via respiratory tract, digestive tract and skin, and havetoxic effect on central nervous system and myocardium. Low concentrationfluoride contamination would result in crispy calcification of teeth andbones. Pollutant Discharge Standard of Ceramic Industry (GB25464-2010)provides that the fluoride discharge standard must be less than 5.0mg/m³, while using a fluoride as low temperature sintering aid for amagnesium oxide ceramic would inevitably increase discharge of gaseousfluorides and increase the burden of environmental protectioninvestment. Furthermore, the fluoride particles in the residualsolubilized fluorides in the ceramic are present in the form ofsubstituted oxygen ions, which would result in decrease of chemicalstability of intergranular bonds and difficulty of resisting long periodof erosion by the flux inclusions in the magnesium melt. In the slurryfor preparing the ceramic foam filter disclosed in Chinese patentdocument CN 101,138,691 A, sodium silicate, a silica sol and ethylsilicate are used as adhesives, and the component SiO₂ present betweenthe sintered foam ceramic particles are prone to react with the melt ofmagnesium and the alloys thereof in accordance with equation (4), whichalso decreases the chemical stability of the ceramic foam. In theChinese patent documents CN 100, 536,986 C (Magnesium Oxide Ceramic FoamFilter) and CN 103,553,686 A (A Magnesium-Alumina Spinel Ceramic FoamFilter and Method of Preparing the Same), diboron trioxide and sodiumborate are used as the low temperature sintering aids for the magnesiumoxide ceramic. Diboron trioxide will form a liquid at a temperaturehigher than 450° C., and when sintering temperature is higher than 1350°C., it will react with magnesium oxide to form magnesium borate presentin a liquid form, so as to decrease the sintering temperature. However,diboron trioxide is prone to react with magnesium and aluminum, and thusis not stable in the magnesium alloy or aluminum alloy melt. Moreover,since diboron trioxide can be dissolved in a solvent, such as water andethanol, it can absorb water in the air mightily to form boric acid. Thediboron trioxide added in the process of preparation of a ceramic foamwill be dissolved in water to form a water solution of boric acid, whichis prone to react with magnesium oxide to form a magnesium borateprecipitate, so as to reduce its effect. Gallium oxide is a family oxideof diboron trioxide, and forms a spinel-type MgGa₂O₄ with magnesiumoxide at a low temperature, so as to decrease the sintering temperature.However, gallium has rare resources (gallium is a strategic reservedmetal), and the higher cost has limited its use in general ceramics.

SUMMARY OF THE INVENTION

The present invention provides an MA-M₂T spinel solidsolution-reinforced magnesium oxide-based ceramic foam filter that canbe sintered at low temperature, has excellent chemical stability andthermal shock resistance, and a method for preparing the same.

In order to achieve the above-identified technical objective, thetechnical solution of the present invention are as follows:

An MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramicfoam filter that is obtained by coating onto a polyurethane foam carriera slurry of a magnesium oxide-based ceramic comprising a nanometertitanium oxide sintering aid, and then drying and sintering.

A method for preparing a spinel solid solution-reinforced magnesiumoxide-based ceramic foam filter comprising the steps of:

(1) Preparing a ceramic slurry having a solid content of 60%-70% bydosing 15%-25% by mass of a nanometer alumina sol, 0.8%-1.5% by mass ofa rheological agent, and the balance magnesium oxide ceramic powdercomprising a nanometer titanium oxide sintering aid, and then addingdeionized water and ball milling to mix until uniform, and vacuumdegassing. The added nanometer alumina sol forms γ-Al₂O₃ coating film onthe surfaces of the light calcined magnesium oxide particles and thehighly uniformly dispersed nanometer TiO₂ powder, and in the sinteringprocess, γ-Al₂O₃ in the alumina sol will perform an in-situ reactionwith MgO in contact with it and form a Mg—Al spinel phase (MgAl₂O₄, MA),MA and M₂T are completely dissolved with each other at a temperatureabove 1350° C. The XRD analysis results show that the ceramic foamfilter prepared in accordance with the present invention comprises onlypericlase MgO and MA-M₂T spinel solid solution phases.

The rheological agent is a mixture of polyacrylic acid and a celluloseether, wherein polyacrylic acid accounts for 20% of the mass of therheological agent, the cellulose ether is one of industrially usedhydroxyethyl cellulose and hydroxy propyl methyl cellulose, or a mixturethereof. The Cellulose ether and polyacrylic acid are not only a gooddispersant for the nanometer titanium oxide powder which can prevent theslurry from agglomeration, but also functions as an adhesive inpreparing a biscuit. Upon soaking, the slurry can be securely adhered toa polyurethane foam template such that the biscuit will have a very bigstrength, and the cellulose ether and polyacrylic acid can escape veryeasily in the sintering process without contaminating the articles, soas to ensure the quality of the ceramic foam filter.

The magnesium oxide ceramic powder comprising a nanometer titanium oxidesintering aid is a mixture of a magnesium oxide powder and a nanometertitanium oxide powder. By adding nanometer titanium oxide to the ceramiccomponent, as a changeable electrovalence oxide, Ti⁴⁺ will be dispersedinto the crystal of MgO, which increases defects in the crystal (suchas, for example, vacancies) that activate the lattice, and forms a solidsolution that Mg²⁺ ions are substituted by Ti⁴⁺ ions, therebyfacilitating direct intergranular bonding. The lattice constant of Al₂O₃is close to that of MgO, and in the sintering process, Al₂O₃ can besolid-solubilized to the lattices of MgO and cause a lattice distortionto the MgO crystal. The diffusion rate of Al³⁺ is small, and thesolubility of Al₂O₃ in periclase MgO is very small (only 3% at 1700°C.). The sintering process is primarily to form a new compound MA phasethrough sintering reactions, and a limited solid solution between the MAphase and MgO, so as to facilitate the sintering and the bonding of theparticles. Nanometer powder means a super-fine powder having a particlesize less than 100 nm, and has the characteristics of high specificsurface area, high surface energy and high activity, among others.Therefore, a nanometer powder can be readily bonded with other atoms,and the melting point and sintering temperature would be much lower thana fine powder. The sintering aid added in the form of nanometer titaniumoxide and nanometer alumina sol can fill the space between the finepowder particles, which optimizes the match and mixing uniformity ofceramic particles. Furthermore, due to the nanometer powder's surfaceand interface effect, the full contact of the nanometer titanium oxideand the highly active nanometer γ-Al₂O₃ in the alumina sol with the MgOparticles will increase the reaction speed quickly, thereby decreasingthe sintering temperature and increasing the density and mechanicalproperties thereof, and the decrease of the sintering temperature willbe helpful in reducing the energy consumption and the production cost ofthe ceramic foam filter. The magnesium oxide powder is a fused magnesiumoxide with a high anti-hydration ability and a particle size in theorder of 250-500 meshes (mean diameter d₅₀ being 25 μm-58 μm). Theparticulate fused magnesium oxide is wrapped by continuous nanometeralumina sol film and in close contact with highly dispersed nanometertitanium oxide particles, which in the sintering process perform in-situreactions with MgO to form a Mg—Al spinel MA phase and a Mg—Ti spinelM₂T phase, which are fully dissolved with each other at a temperatureabove 1350° C., directly weld the MgO particles together in thesintering process and form intergranular secondary spinels(intergranular spinel) M₂T and MA by exsolution precipitation uponcooling, which may compensate for the stress at the phase interfaceswhich results in a stress relaxation of the material when cooled aftersintering. Furthermore, the pinning effect of the spinel solid solutionphases will inhibit the fast growth of MgO particles, so as to refinethe microstructure of the ceramic foam and increase the compactness ofthe ceramic.

The nanometer titanium oxide powder accounts for 1%-2% by mass of theceramic powder, and has a particle size of 30-60 nm.

Preferably, the nanometer alumina sol has a solid content of 20%-25%.

A method of preparing the ceramic slurry is: adding the fused magnesiumoxide powder into a ball milling tank in accordance with the ratio:making a solution using the nanometer alumina sol, the rheological agentand deionized water and added therein the nanometer titanium oxidepowder, ultrasonic treating the mixture for 30-60 minutes to cause thenanometer titanium oxide powder to be fully dispersed in the solution;adding the mixture into the ball milling tank; adding corundum balls ina ball to material ratio of 2:1; and ball milling for 2-4 hours with arotation speed of 60-120 rpm until a uniform mixture is achieved; andthen vacuum degassing for 10-15 minutes at a negative pressure of 0.02MPa-0.05 MPa.

(2) Soaking a polyurethane foam template into the ceramic slurry,squeezing by a roller press the polyurethane foam template to removeredundant slurry therein to make a biscuit, and heating the biscuit to atemperature of 80° C.-120° C. to dry the biscuit.

The specification of the polyurethane foam template is 10 PPI-20 PPI(Pores per inch); the polyurethane foam template is first soaked in awater solution of 15%-20% NaOH at 40° C.-50° C. for surface etching for40-60 minutes, washed by clean water and naturally dried, and thensoaked into a water solution of 2%-4% wetting agent dodecylbenzenesulfonic acid, and taken out and dried before use. By NaOH etching, thesurface of the polyurethane foam template is roughened, and aftertreated by the water solution of the wetting agent dodecylbenzenesulfonic acid, it will be easy for the ceramic slurry to be coated ontothe polyurethane foam template.

(3) Putting the dried biscuit into a sintering furnace, elevating thetemperature to 1400° C.-1600° C. for high temperature sintering, coolingto the room temperature with the furnace to obtain a magnesiumoxide-based ceramic foam filter.

The sintering process is: the temperature is elevated to 550° C. at atemperature rising rate of 30° C./h to have organic substances (forexample, the polyurethane foam and the rheological agent, among others)in the ceramic foam filter biscuit to be decomposed, gasified anddischarged, then the temperature is elevated to 1100° C. at atemperature rising rate of 200° C./h. In the low temperature sinteringstage, a lower temperature rising rate would be able to prevent thepolyurethane foam and the rheological agent from being decomposed tooquickly, which would cause the biscuit to collapse or be damaged due todeformation. Finally, the temperature is elevated to 1400° C.-1600° C.at a temperature rising rate of 50° C./h and the temperature ismaintained for 2-3 hours. In the high temperature sintering stage, afterthe sintering temperature is above 1100° C., the lower temperaturerising rate would be able to ensure a consistent temperature in thesintered body, and avoid a constant spinel formation rate and sinteredbody deformation and cracking due to extremely quick generation of phasetransformation stresses.

The method for preparing the magnesium oxide-based ceramic foam filterprovided by the present invention is simple, cost low, efficiency highand suitable for industrial scale production. The magnesium oxide-basedceramic foam filter so prepared is free of any components that decreaseits chemical stability. The nanometer alumina sol can function as anadhesive, and the highly uniformly dispersed nanometer alumina sol andthe nanometer titanium oxide react with the magnesium oxide particles toform an MA-M₂T spinel solid solution that are chemically stable againstthe melt of magnesium and alloys thereof, and weld the magnesium oxideparticles together. Therefore, the ceramic foam filter exhibits goodstrength, chemical stability and thermal shock resistance, and isespecially useful for filtration and purification of the inclusions inthe melt of magnesium and alloys thereof. It can also be used forfiltration and purification of the melt of aluminum and alloys thereof.As compared to the prior art, the present invention achieves thefollowing technical effects:

1. The MA-M₂T spinel solid solution-reinforced magnesium oxide ceramicfoam filter of the present invention exhibits excellent chemicalstability. TiO₂ essentially does not have the solid solution function inMgO. The added nanometer titanium oxide of the technical solution of thepresent invention will facilitate the sintering of MgO and react withMgO to form a Mg—Ti spinel (Mg₂TiO₄, M₂T) phase that exhibits highchemical stability. Although the starting material alumina sol componentcomprises γ-Al₂O₃ that will react with the magnesium melt, the addednanometer alumina sol forms γ-Al₂O₃ coating film on the surfaces of thelight calcined magnesium oxide particles and the highly uniformlydispersed nanometer TiO₂ powder, and in the sintering process, γ-Al₂O₃in the alumina sol will perform an in-situ reaction with MgO in contactwith it and form a Mg—Al spinel phase (MgAl₂O₄, MA). MA and M₂T arecompletely dissolved with each other at a temperature above 1350° C. TheXRD analysis results show that the ceramic foam filter prepared inaccordance with the present invention comprises only periclase MgO andMA-M₂T spinel solid solution phases.

In the reaction system of MgO-Al₂O₃ sintered ceramic with magnesium meltand addition of aluminum oxide, the following reactions may be presentin addition to reaction equation (1):

3Mg_((l)+)4Al₂O_(3(s)=)3MgAl₂O_(4(s)+)2Al_((l))   (5)

The reaction of aluminum oxide with magnesium oxide to form Mg—Al spinelMgAl₂O₄ is:

MgO_((s)+)Al₂O_(3(s)=)MgAl₂O_(4(s))   (6)

The reaction of magnesium melt with the Mg—Al spinel MgAl₂O₄ is:

3Mg_((l)+)MgAl₂O_(4(s)=)2Al_((l)+)4MgO_((s))   (7)

According to Pure substance Thermochemical Data Handbook (Edited byBarin Ihsan, Translated by Nailiang Cheng, Beijing: The Science Press,2003), at 900-1200 K, the Gibbs free energies of the substances in thereaction system of magnesium melt with Mg—Al spinel MgAl₂O₄ and thecalculated results of the Gibbs free energy changes ΔG₁, ΔG₅, ΔG₆, ΔG₇of the reactions (1), (5), (6) and (7) are shown in Table 1.

TABLE 1 The calculated results of the Gibbs free energy changes ΔG forthe reactions in the reaction system of magnesium melt with Mg—Al spinelat 900-1200 K Gibbs Free Energy of Substance G (kJ/mol) Gibbs FreeEnergy AG of Reaction (kJ/mol) T (K) Mg Al MgO γ-Al₂O₃ MgAl₂O₄ ΔG₁ ΔG₅ΔG₆ ΔG₇ 900 −39.937 −35.835 −642.540 −1744.794 −2428.590 −134.685−258.453 −41.256 −93.429 1000 −47.208 −42.645 −650.503 −1762.887−2454.192 −132.288 −254.694 −40.802 −91.486 1100 −55.019 −50.158−658.978 −1782.290 −2481.564 −129.903 −250.791 −40.296 −89.607 1200−63.127 −57.960 −667.925 −1802.900 −2510.578 −127.414 −246.673 −39.753−87.661

The Gibbs free energy ΔG₅ for the reaction equation (5), i.e., magnesiummelt reacts with aluminum oxide to form Mg—Al spinel, is always thesmallest at different temperatures, indicating that this reaction willhappen first at common smelting and refining temperatures of magnesiumalloys. Although reaction equation (7), i.e., liquid magnesium reactswith Mg—Al spinel, may be possible thermodynamically, this reaction issubstantially a reaction between the liquid magnesium and the decomposedproduct of Mg—Al spinel, i.e., aluminum oxide. However, it can be knownfrom Table 1 that the reaction of Mg—Al spinel being decomposed intoaluminum oxide and magnesium oxide (the reverse reaction of reactionequation (6)) would be hard to happen, and the residual aluminum oxidein the sintered ceramic will also react with the liquid magnesium firstin accordance with reaction equation (5) to form Mg—Al spinel. On theother hand, in the MgO—Al₂O₃ phase diagram, the MgO side is an eutecticphase diagram of MgO solid solution and MA spinel solid solution, and inthe process of forming MA through in-situ reaction, there is essentiallyno dispersion of O²⁻, but only interdiffusion of Mg²⁺ and Al³⁺ via fixedoxygen lattices, and the speed of formation is determined by Al³⁺ thatis with a slower diffusion. The MA phase is formed primarily on theAl₂O₃ side by endotaxy growth, resulting in formation of limited solidsolution between MA phase and MgO, and the MgO content in the exteriorMA layer contacting the MgO particles is much higher than the averagethereof. Since MgO does not react with Mg melt, the Mg—Al spinel phasein the sintered ceramic structure which welds the MgO particles togetherwill be able to present stably in the magnesium melt.

The MA-M₂T spinel solid solution-reinforced magnesium oxide-based foamceramic filter of the present invention does not comprise any componentsthat decrease its chemical stability. The added nanometer alumina solnot only forms γ-Al₂O₃ coating film on the surfaces of the lightcalcined magnesium oxide particles and the highly uniformly dispersednanometer TiO₂ powder and functions as an adhesive, in the sinteringprocess, Al₂O₃ and TiO₂ will perform an in-situ reaction with MgO andform an MA-M₂T spinel solid solution phase, and thus avoids the damageto the chemical stability of the ceramic foam due to addition ofadhesives such as silica sol, ethyl silicate, among others, to the priorart products. Furthermore, the ceramic components are free of sodiumsalts (for example, sodium carboxymethyl cellulose is not used in therheological agent), which removes the obstacle for the ceramic sinteringwhich is caused by residual Na⁺ having a greater ion radius.

Since the reaction equations (1) and (5) can take place spontaneously atthe commonly used smelting temperatures, and the smelting temperaturesof aluminum and alloys thereof are the same as the smelting temperaturesof magnesium and alloys thereof, adverse reactions of equations (1) and(5) will not take place between MgO, MA spinel phase and the melt ofaluminum and alloys thereof. As with the melt of magnesium and alloysthereof, this avoids the damage to the chemical stability of ceramicfoam in the melt of aluminum and alloys thereof due to addition ofadhesives such as silica sol, ethyl silicate, among others. Even if thematerials comprise 1% of SiO₂, the reaction of Al+SiO₂→Al₂O₃+Si willtake place at a high temperature between the melt of aluminum and alloysthereof and SiO₂ in the ceramic. Therefore, the MA-M₂T spinel solidsolution-reinforced magnesium oxide ceramic foam filter of the presentinvention can also be used in the smelting and purification of aluminumand alloys thereof.

2. The MA-M₂T spinel solid solution-reinforced magnesium oxide ceramicfoam filter of the present invention has good low temperature sinteringperformance. In the technical solution of the present invention,nanometer titanium oxide is added to the ceramic component. As achangeable electrovalence oxide, Ti⁴⁺ will be dispersed into the crystalof MgO, which increases defects in the crystal (such as, for example,vacancies) that activate the lattice, and forms a solid solution thatMg²⁺ ions are substituted by Ti⁴⁺ ions, thereby facilitating directintergranular bonding. The lattice constant of Al₂O₃ is close to that ofMgO, and in the sintering process, Al₂O₃ can be solid-solubilized to thelattices of MgO and cause a lattice distortion to the MgO crystal. Thediffusion rate of Al³⁺ is small, and the solubility of Al₂O₃ inpericlase MgO is very small (only 3% at 1700° C.). The sintering processis primarily to form a new compound MA phase through sinteringreactions, and a limited solid solution between the MA phase and MgO, soas to facilitate the sintering and the bonding of the particles.Nanometer powder means a super-fine powder having a particle size lessthan 100 nm, and has the characteristics of high specific surface area,high surface energy and high activity, among others. Therefore,nanometer powder can be readily bonded to other atoms, and the meltingpoint and sintering temperature would be much lower than a fine powder.The sintering aid added in the form of nanometer titanium oxide andnanometer alumina sol can fill the space between the fine powderparticles, which optimizes the match and mixing uniformity of ceramicparticles. Furthermore, due to the nanometer powder's surface andinterface effect, the full contact of the nanometer titanium oxide andthe highly active nanometer γ-Al₂O₃ in the alumina sol with the MgOparticles will increase the reaction speed quickly, thereby decreasingthe sintering temperature and increasing the density and mechanicalproperties thereof, and the decrease of the sintering temperature willbe helpful in reducing the energy consumption and the production cost ofthe ceramic foam filter. The experimental results show that, when thesintering temperature is below 1400° C., the bonding of the sinteringstructure between the MgO particles is not sufficient, resulting in alower strength of the product. The sintering temperature of thestructurally well bonded MA-M₂T spinel solid solution-reinforcedmagnesium oxide foam ceramic filter is 1400° C.-1600° C.

3. The MA-M₂T spinel solid solution-reinforced magnesium oxide ceramicfoam filter of the present invention has good thermal shock resistance.The present invention uses fused magnesium oxide that has highanti-hydration ability. Since the formation of MA by sintering throughthe reaction of magnesium oxide and aluminum oxide will cause a volumeexpansion (linear expansion rate being 2.3%; volume expansion rate being6.9%), it increases the sintering burden. The metastable transitionalcrystal structure that Al³⁺ is distributed irregularly in the octahedraland tetrahedral voids formed by oxygen ions is the same with the Mg—Alspinel MA crystal structure. By substituting γ-Al₂O₃ for α-Al₂O₃, thesintering characteristics of MgO-MA material will be changed wherein thevolume of MA will contract for 2.7%, thereby increasing the sinteringdensity. In the solution provided in the present invention, theparticulate fused magnesium oxide is wrapped by continuous nanometeralumina sol film and in close contact with the highly dispersednanometer titanium oxide particles, which in the sintering processperform in-situ reactions with MgO to form a Mg—Al spinel MA phase and aMg—Ti spinel M₂T phase, which are fully dissolved with each other at atemperature above 1350° C., directly weld the MgO particles together inthe sintering process and form intergranular secondary spinels(intergranular spinel) M₂T and MA by exsolution precipitation uponcooling, which may compensate for the stress at the phase interfaceswhich results in a stress relaxation of the material when cooled aftersintering. Furthermore, the pinning effect of the spinel solid solutionphases will inhibit the fast growth of MgO particles, so as to refinethe microstructure of the ceramic foam and increase the compactness ofthe ceramic. Compared to MgO and Al₂O₃, the spinel phase has a smallthermal expansion factor and a low thermal conductivity. Therefore, theMA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramicfoam filter so prepared has relatively high mechanical properties, hightemperature impact resistance and thermal shock resistance.

Furthermore, the polyurethane foam plastic is a surface inactivehydrophobic material and has poor wettability with respect to theceramic slurry. Thus, this would affect the slurry coating performanceof the template. In the preparation method of the present invention, theetching of the polyurethane foam plastic template by NaOH makes thesurface thereof roughened, and together with the treatment by a watersolution of wetting agent dodecyl benzene sulfonate, the ceramic slurrycan be easily coated to the polyurethane foam template. Additionally,the rheological agent, i.e., the cellulose ether and polyacrylic acid,is not only a good dispersant for the nanometer titanium oxide powderwhich prevents the slurry from being agglomerated, but also serves as anadhesive in the preparation of the biscuit. Upon soaking, the slurry canbe securely adhered to the polyurethane foam template, which imparts thebiscuit a very big strength, and the rheological agent can escape veryeasily in the sintering process without contaminating the article.Therefore, the quality of the ceramic foam filter is guaranteed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the flow diagram of the process for preparing the MA-M₂Tspinel solid solution-reinforced magnesium oxide-based foam ceramicfilter.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The present invention will be further described by referring to theaccompany drawing and embodiments.

The MA-M₂T spinel solid solution-reinforced magnesium oxide-based foamceramic filter is obtained by coating onto a polyurethane foam carrier aslurry of light calcined magnesium oxide-based ceramic comprising ananometer titanium oxide sintering aid, and then drying and sintering.The detailed process of preparation is shown in FIG. 1.

EXAMPLE 1

With the ratio that nanometer titanium oxide accounts for 1% by mass ofceramic powder, a nanometer titanium oxide having a particle size of 30nm and a fused magnesium oxide powder having a particle size of theorder of 250 meshes (mean size d₅₀ being 58 μm) were dosed to prepare aceramic powder. With a mass ratio of 1:4 (polyacrylic acid: hydroxyethylcellulose), a rheological agent was prepared by polyacrylic acid andhydroxyethyl cellulose.

With the following mass percentages, 15% nanometer alumina sol having asolid content of 20% (a commercial nanometer alumina sol having a pHvalue of about neutral was selected, the same for the followingExamples), 0.8% rheological agent and the balance ceramic powder weredosed. In accordance with the ratio, the fused magnesium oxide powderwas first added into a ball milling tank; the nanometer alumina sol, therheological agent and a suitable amount of deionized water (the amountto be determined by the solid content of the ceramic slurry, the samefor the following Examples) were mixed and subjected to an ultrasonictreatment for 30 minutes to have the nanometer titanium oxide powderfully dispensed prior to being added to the ball milling tank; with aratio of 2:1 of ball to material, corundum balls were added, and ballmilling was performed for 4 hours with a rotation speed of 60 rpm toachieve a uniform mixture, and then vacuum degassing for 15 minutes at anegative pressure 0.02 MPa to obtain a ceramic slurry having a solidcontent of 60%.

A 10 PPI polyurethane foam plastic template was subjected to a surfaceetching for 60 minutes in a 40° C. water solution of 15% NaOH, and thenwashed by clean water and dried naturally. Next, the template was soakedin a water solution of 2% dodecylbenzene sulfonic acid wetting agent,and then taken out and dried. The polyurethane foam plastic template wasthen soaked in the ceramic slurry, and a roller press was used tosqueeze the polyurethane foam plastic template to remove the redundantslurry to form a biscuit. The biscuit was then dried by heating it to80° C.

The dried biscuit was put in a sintering furnace, and the temperaturewas elevated to 550° C. at a temperature rising rate of 30° C./h to haveorganic substances such as the polyurethane foam and rheological agent,among others, in the ceramic foam filter biscuit decomposed, gasifiedand discharged, then the temperature was elevated to 1100° C. at atemperature rising rate of 200° C./h. Finally, the temperature waselevated to 1600° C. at a temperature rising rate of 50° C./h and thetemperature was maintained for 2.5 hours. The biscuit was then cooled tothe room temperature with the furnace to obtain a magnesium oxide-basedceramic foam filter.

EXAMPLE 2

With the ratio that nanometer titanium oxide accounts for 2% by mass ofceramic powder, a nanometer titanium oxide having a particle size of 60nm and a fused magnesium oxide powder having a particle size of theorder of 500 meshes (mean size d₅₀ being 25 μm) were dosed to prepare aceramic powder. With a mass ratio of 1:4 (polyacrylic acid:hydroxypropyl methyl cellulose), a rheological agent was prepared bypolyacrylic acid and hydroxypropyl methyl cellulose.

With the following mass percentages, 20% nanometer alumina sol having asolid content of 25%, 1.5% rheological agent and the balance ceramicpowder were dosed. In accordance with the ratio, the fused magnesiumoxide powder was first added into a ball milling tank; the nanometeralumina sol, the rheological agent and a suitable amount of deionizedwater were mixed and subjected to an ultrasonic treatment for 45 minutesto have the nanometer titanium oxide powder fully dispensed prior tobeing added to the ball milling tank; with a ratio of 2:1of ball tomaterial, corundum balls were added, and ball milling was performed for2 hours with a rotation speed of 120 rpm to achieve a uniform mixture,and then vacuum degassing for 10 minutes at a negative pressure 0.05 MPato obtain a ceramic slurry having a solid content of 65%.

A 20 PPI polyurethane foam plastic template was subjected to a surfaceetching for 40 minutes in a 50° C. water solution of 20% NaOH, and thenwashed by clean water and dried naturally. Next, the template was soakedin a 4% water solution of dodecylbenzene sulfonic acid wetting agent,and then taken out and dried. The polyurethane foam plastic template wasthen soaked in the ceramic slurry, and a roller press was used tosqueeze the polyurethane foam plastic template to remove the redundantslurry to form a biscuit. The biscuit was then dried by heating it to120° C.

The dried biscuit was put in a sintering furnace, and the temperaturewas elevated to 550° C. at a temperature rising rate of 30° C./h to haveorganic substances such as the polyurethane foam and rheological agent,among others, in the ceramic foam filter biscuit decomposed, gasifiedand discharged, then the temperature was elevated to 1100° C. at atemperature rising rate of 200° C./h. Finally, the temperature waselevated to 1400° C. at a temperature rising rate of 50° C./h and thetemperature was maintained for 3 hours. The biscuit was then cooled tothe room temperature with the furnace to obtain a magnesium oxide-basedceramic foam filter.

EXAMPLE 3

With the ratio that nanometer titanium oxide accounts for 1.5% by massof ceramic powder, a nanometer titanium oxide having a particle size of50 nm and a fused magnesium oxide powder having a particle size of theorder of 325 meshes (mean size d₅₀ being 45 μm) were dosed to prepare aceramic powder. With a mass ratio of 1:4 (polyacrylic acid: hydroxyethylcellulose), a rheological agent was prepared by polyacrylic acid andhydroxyethyl cellulose.

With the following mass percentages, 25% nanometer alumina sol having asolid content of 22%, 1.0% rheological agent and the balance ceramicpowder were dosed. In accordance with the ratio, the fused magnesiumoxide powder was first added into a ball milling tank; the nanometeralumina sol, the rheological agent and a suitable amount of deionizedwater were mixed and subjected to an ultrasonic treatment for 60 minutesto have the nanometer titanium oxide powder fully dispensed prior tobeing added to the ball milling tank; with a ratio of 2:1 of ball tomaterial, corundum balls were added, and ball milling was performed for3 hours with a rotation speed of 90 rpm to achieve a uniform mixture,and then vacuum degassing for 12 minutes at a negative pressure 0.03 MPato obtain a ceramic slurry having a solid content of 70%.

A 15 PPI polyurethane foam plastic template was subjected to a surfaceetching for 50 minutes in a 45° C. water solution of 18% NaOH, and thenwashed by clean water and dried naturally. Next, the template was soakedin a 3% water solution of dodecylbenzene sulfonic acid wetting agent,and then taken out and dried. The polyurethane foam plastic template wasthen soaked in the ceramic slurry, and a roller press was used tosqueeze the polyurethane foam plastic template to remove the redundantslurry to form a biscuit. The biscuit was then dried by heating it to100° C.

The dried biscuit was put in a sintering furnace, and the temperaturewas elevated to 550° C. at a temperature rising rate of 30° C./h to haveorganic substances such as the polyurethane foam and rheological agent,among others, in the ceramic foam filter biscuit decomposed, gasifiedand discharged, then the temperature was elevated to 1100° C. at atemperature rising rate of 200° C./h. Finally, the temperature waselevated to 1500° C. at a temperature rising rate of 50° C./h and thetemperature was maintained for 2 hours. The biscuit was then cooled tothe room temperature with the furnace to obtain a magnesium oxide-basedceramic foam filter.

EXAMPLE 4

With the ratio that nanometer titanium oxide accounts for 2% by mass ofceramic powder, a nanometer titanium oxide having a particle size of 60nm and a fused magnesium oxide powder having a particle size of theorder of 300 meshes (mean size d₅₀ being 48 μm) were dosed to prepare aceramic powder. With a mass ratio of 2:4:4 (polyacrylic acid:hydroxypropyl methyl cellulose:hydroxyethyl cellulose), a rheologicalagent was prepared by polyacrylic acid, hydroxypropyl methyl celluloseand hydroxyethyl cellulose.

With the following mass percentages, 20% nanometer alumina sol having asolid content of 20%, 1.0% rheological agent and the balance ceramicpowder were dosed. In accordance with the ratio, the fused magnesiumoxide powder was first added into a ball milling tank; the nanometeralumina sol, the rheological agent and a suitable amount of deionizedwater were mixed and subjected to an ultrasonic treatment for 45 minutesto have the nanometer titanium oxide powder fully dispensed prior tobeing added to the ball milling tank; with a ratio of 2:1 of ball tomaterial, corundum balls were added, and ball milling was performed for3 hours with a rotation speed of 100 rpm to achieve a uniform mixture,and then vacuum degassing for 15 minutes at a negative pressure 0.02 MPato obtain a ceramic slurry having a solid content of 65%.

A 15 PPI polyurethane foam plastic template was subjected to a surfaceetching for 50 minutes in a 45° C. water solution of 15% NaOH, and thenwashed by clean water and dried naturally. Next, the template was soakedin a water solution of 4% dodecylbenzene sulfonic acid wetting agent,and then taken out and dried. The polyurethane foam plastic template wasthen soaked in the ceramic slurry, and a roller press was used tosqueeze the polyurethane foam plastic template to remove the redundantslurry to form a biscuit. The biscuit was then dried by heating it to100° C.

The dried biscuit was put in a sintering furnace, and the temperaturewas elevated to 550° C. at a temperature rising rate of 30° C./h to haveorganic substances such as the polyurethane foam and rheological agent,among others, in the ceramic foam filter biscuit decomposed, gasifiedand discharged, then the temperature was elevated to 1100° C. at atemperature rising rate of 200° C./h. Finally, the temperature waselevated to 1550° C. at a temperature rising rate of 50° C./h and thetemperature was maintained for 2 hours. The biscuit was then cooled tothe room temperature with the furnace to obtain a magnesium oxide-basedceramic foam filter.

In the above Examples, the experiments show that the ceramic foamsprepared exhibited excellent thermal shock resistance and strength. Theceramic foams did not exhibit cracking after 50 times cooling in 900° C.air, and the natural temperature strength of the 10 PPI ceramic foamfilters with a size of 75 mm×75 mm×25 mm was not less than 2 MPa.

The Examples described above would not limit the present invention inany way. All technical solutions obtained by equivalent substitutions ortransformations are within the scope of present invention.

What is claimed is:
 1. An MA-M₂T spinet solid solution-reinforced magnesium oxide-based ceramic foam filter wherein the filter is obtained by coating onto a polyurethane foam carrier a slurry of a magnesium oxide-based ceramic comprising a nanometer titanium oxide sintering aid, and then drying and sintering.
 2. A method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter wherein the method comprises the steps of: 1) preparing a ceramic slurry having a solid content of 60 wt. %-70 wt. % by dosing 15%-25% by mass of a nanometer alumina sol, 0.8%-1.5% by mass of a rheological agent, and the balance magnesium oxide ceramic powder comprising a nanometer titanium oxide sintering aid, and then adding deionized water and ball milling to mix until uniform, and vacuum degassing the mixture; the rheological agent is a mixture of polyacrylic acid and a cellulose ether, wherein polyacrylic acid accounts for 20% of the mass of the rheological agent, the cellulose ether is one of industrial-use hydroxyethyl cellulose and hydroxy propyl methyl cellulose, or a mixture thereof; the magnesium oxide ceramic powder comprising a nanometer titanium oxide sintering aid is a mixture of a magnesium oxide powder and a nanometer titanium oxide powder; 2) soaking a polyurethane foam plastic template into the ceramic slurry, squeezing by a roller press the polyurethane foam plastic template to remove redundant slurry to make a biscuit, and drying the biscuit by heating it to 80° C.-120° C.; 3) putting the dried biscuit into a sintering furnace, elevating the temperature to 1400° C.-1600° C. and performing a high temperature sintering, cooling to the room temperature with the furnace to obtain the magnesium oxide-based ceramic foam filter.
 3. The method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter according to claim 2, wherein the nanometer alumina sol has a solid content of 20%-25%.
 4. The method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter according to claim 2, wherein the nanometer titanium oxide powder accounts for 1%-2% by mass of the ceramic powder.
 5. The method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter according to claim 2, wherein the magnesium oxide powder is a fused magnesium oxide powder having a particle size in the order of 250-500 meshes.
 6. The method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter according to claim 2, wherein the nanometer titanium oxide powder has a particle size of 30-60 nm.
 7. The method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter according to claim 2, wherein a method of preparing the ceramic slurry is: with the ratio, adding a fused magnesium oxide powder into a ball milling tank; making a solution using the nanometer alumina sol, the rheological agent and deionized water and added therein the nanometer titanium oxide powder, ultrasonic treating the mixture for 30-60 minutes to have the nanometer titanium oxide powder fully dispersed in the solution; adding the mixture into the ball milling tank; with a ball to material ratio of 2:1, adding corundum balls and ball milling for 2-4 hours with a rotation speed of 60-120 rpm until a uniform mixture is achieved, and then vacuum degassing for 10-15 minutes at a negative pressure of 0.02 MPa-0.05 MPa.
 8. The method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter according to claim 2, wherein the specification of the polyurethane foam template is 10 PPI-20 PPI; the polyurethane foam template is first soaked in a water solution of 15%-20% NaOH at 40° C.-50° C. for surface etching for 40-60 minutes, washed by clean water and naturally dried, and then soaked into a water solution of 2%-4% wetting agent dodecylbenzene sulfonic acid and taken out and dried.
 9. The method for preparing an MA-M₂T spinel solid solution-reinforced magnesium oxide-based ceramic foam filter according to claim 2, wherein in step 3), the sintering process is: the temperature is first elevated to 550° C. at a temperature rising rate of 30° C./h to have organic substances in the ceramic foam filter biscuit decomposed, gasified and discharged, then the temperature is elevated to 1100° C. at a temperature rising rate of 200° C./h, and finally, the temperature is elevated to 1400° C.-1600° C. at a temperature rising rate of 50° C./h and the temperature is maintained for 2-3 hours. 